No word yet on what Seth will be doing at Yahoo (YHOO). UPDATE: Seth’s new position will be senior vice president, search products team, Yahoo announced on a blog post.

In the letter announcing his departure, below, he cites the new job’s strain on his family–it’s worth noting that his description of the situation is a bit more impassioned than the generic “leaving to spend time with his family” explanation, though not really more detailed.

Here’s what Jeff Levick, president of global advertising and strategy at AOL, said about Seth in September:

“Shashi is unmatched in the industry as an innovator with an outstanding track record of developing new and better ways to serve advertisers on the Web. As we move forward on our strategy of becoming the world’s largest provider of display advertising, Shashi will play a critical role in creating the best products in the business for our advertising partners.”

And here’s what Levick had to say about him today, followed by Shashi’s letter to his former colleagues:

From: Levick, Jeff

Sent: Wednesday, January 13, 2010 3:12 PM

To: AOL_Advertising

Subject: AOL Advertising Organization Changes

AOL Advertising–Along with the other changes going on across the company today, I wanted to let you know Shashi Seth will also be leaving AOL effective today. While Shashi was only with us for 90 days, we appreciate the early steps he took to help us redefine AOL’s advertising product roadmap.? ?I’ve attached Shashi’s note below. As you’ll see he’s taken another position in the Valley–at Yahoo–which will enable him to remain closer to his family.

In the interim, Jamie Fellows will be leading our Global Advertising Products organization. I’ll look forward to seeing everyone at our All Hands next Tuesday and as always if you have questions, please let me know.

Best, ?Jeff

Dear Team:

It is with a heavy heart that I must tell you that I am resigning from my role at AOL. Very recent developments in my personal life have made it impossible for me to maintain an executive role that requires extensive travel.

Over the last 3 months my relationship with my wife has been impacted severely, and the last 2 months have been very difficult for me as I saw my family unravel in front of my eyes. My family has always come first, and decided that we needed to give it everything we have to put this family back together. Spending more time together will be an important ingredient for success. The holidays gave us an opportunity to start that process, discuss plans for the future and laid out a plan that allows us to reset and rebuild our lives.

Last week I was approached by a Bay Area company about an interesting role, discussions progressed very quickly, and I’ve accepted their offer to join. This new role will enable me to remain local, and gives me lots of flexibility in where I work from (we might move to India for some time) and be available for my family, yet gives me something interesting and challenging to work on.

This is very hard for me, as I have enjoyed working with all of you, and truly believe in our products, vision and AOL as a company. The new organization, the new culture, the new focus, but above all having leaders like Tim and Jeff, all point to success in short order.

I apologize for all the inconvenience and the disruption caused by my action, and I hope you will understand. I will root for you from the sidelines, and celebrate AOL’s success. Someday, we may have the chance to work together again.??Jan 22nd will be my last day at work. Jeff and I have put together a transition plan that will allow us to continue business as usual. You will see emails and meeting invites that will start the process.??I will truly miss working with each and everyone of you.

I think the NSA has a job to do and we need the NSA. But as (physicist) Robert Oppenheimer said, “When you see something that is technically sweet, you go ahead and do it and argue about what to do about it only after you’ve had your technical success. That is the way it was with the atomic bomb.”

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